Pillar 3: Matter and energy 

3.1 Theory 
Philosophy of physics
3.2 Principles
Physics
3.3 Substances
Chemistry

3.1 Theory 
   Meta-knowledge of the study

  • Philosophy of physics (W)
    Philosophical aspects of the physical universe

3.2 Principles
    Principles underlying the physical universe

  • Physics (W)
    Basic laws of the physical world

  Theory
      Meta-knowledge of the study of physics

    •• Philosophy of physics (W)
       Philosophical aspects physics

    •• History of physics (W)
       The story of studing the physical world

  Branches
     Subfields of physics

    •• Mechanics (W)
       Motion, forces and interactions among objects

     ••• Classical mechanics (W)
          Macroscopic bodies at lower speeds

     ••• Quantum mechanics (W)
          Microscopic bodies at high speeds

     ••• Relativistic mechanics (W)
          Mechanics compatible with the theory of relativity

    •• Thermodynamics (W)

     ••• Classical thermodynamics (W)
    The part developed in the 19th century

     ••• Chemical thermodynamics (W)

     ••• Quantum thermodynamics (W)

     ••• Equilibrium thermodynamics (W)

     ••• Non-equilibrium thermodynamics (W)

    •• Statistical physics (W)

    •• Electromagnetism (W)

    •• Optics (W)
       properties of light and interrelations with matter

     ••• Geometrical optics (W)
          Light as rays

     ••• Physical optics (W)
          Light as waves 

     ••• Quantum optics (W)
          Light as particles

    ••Acoustics (W)
       Sound and mechanical waves

    •• Atomic physics (W)
       The structure of atoms and their parts 

    •• Nuclear physics (W)
        Atomic nuclei

    •• Condensed matter physics (W)
       Properties of solid and liquid

    •• Particle physics (W)
    (High energy physics); matter and radiation

    •• Statistical physics (W)

    •• Astrophysics (see astronomy)

    •• Biophysics (see biology)

    •• Geophysics (see geology)

    •• Physical chemistry (see Physical chemistry)

3.3 Substances
   Substances composing the universe

  • Chemistry (W)
    Physical substances and compounds

  Theory

    •• Philosophy of chemistry (W)
        Philosophical aspects of chemistry

    ••  History of chemistry (W)
       The story of chemistry

  Branches

    •• Organic chemistry  (W)
       Organic compounds (containing carbon)

    •• Inorganic chemistry (W)
       Inorganic substances

   •• Physical chemistry (W)
         Physical properties of materials

    •• Analytical chemistry (W)
       Chemical analysis using advanced methods

    •• Biochemistry (W)
        See Biology: Biochemistry

The Matter and energy pillar

The matter and energy pillar explores the basics of the physical universe.
  
3.1

Philosophy of physics has two foci. It explores the philosophical perspectives of the physical universe and it establishes the scientific foundations of modern physics. Philosophy of physics defines key concepts (“matter," “energy," “space," “time," “physical object," and “physical laws”), discusses fundamental issues underlying science (e.g., causality vs. determinism), and establishes valid and reliable research methodologies. While in practice there is one integrated field under the umbrella name “philosophy of physics," this field is composed of two interrelated parts: (1) philosophy of physics₁ (philosophy of the physical universe; see c. 3.1) and philosophy of physics₂ (philosophy of physics as an academic field: see physics).

3.2 

Physics explores the physical universe. Physics is usually divided into two main parts: Classical physics and Modern physics. Classical Physics (W) includes theories and fields that predate the 20th century. Modern physics (W) includes theories and fields from the beginning of the 20th century onwards, including relativity and quantum theory, which changed the face of physics and contributed to the creation of new fields of research. However, the fields associated with classical physics continued to develop during the 20th and 21st centuries. Therefore, the distinction between classical physics and modern physics does not reflect the current state of the physical sciences. 

Physics mapping presents a dilemma between the 'classical-modern' approach and the 'domain based' approach. According to the 'classical-modern' approach, classical mechanics and quantum mechanics are two subfields of physics:
Physics 
1. Classical physics
    1.1 Classical mechanics
    1.2 Modern physics
2. Quantum Mechanics
According to the 'domain-based' approach, classical mechanics and quantum mechanics are two subfields of mechanics:
Physics 
1. Mechanics
    1.1 Classical mechanics
    1.2 Quantum mechanics.
The tree of physics, above, follows the domain-based approach.